add variants of "find visible unit" to game_board

Allow size_t instead of reference to a team, also possibility to
query for presence of a visible unit instead of returning an
iterator to it.

src/game_board.cpp

@@ -61,6 +61,15 @@ unit_map::iterator game_board::find_visible_unit(const map_location &loc,
 	return u;
 }
 
+bool game_board::has_visible_unit(const map_location & loc, const team& current_team, bool see_all)
+{
+	if (!map_.on_board(loc)) return false;
+	unit_map::iterator u = units_.find(loc);
+	if (!u.valid() || !u->is_visible_to_team(current_team, see_all))
+		return false;
+	return true;
+}
+
 unit* game_board::get_visible_unit(const map_location &loc,
 	const team &current_team, bool see_all)
 {

src/game_board.hpp

@@ -117,6 +117,10 @@ class game_board {
 	// Global accessor from unit.hpp
 
 	unit_map::iterator find_visible_unit(const map_location &loc, const team& current_team, bool see_all = false);
+	unit_map::iterator find_visible_unit(const map_location & loc, size_t team, bool see_all = false) { return find_visible_unit(loc, teams_[team], see_all); }
+	bool has_visible_unit (const map_location & loc, const team & team, bool see_all = false);
+	bool has_visible_unit (const map_location & loc, size_t team, bool see_all = false) { return has_visible_unit(loc, teams_[team], see_all); }
+
 	unit* get_visible_unit(const map_location &loc, const team &current_team, bool see_all = false); //TODO: can this not return a pointer?
 };